Fiber optic Distributed Temperature Sensing (DTS) systems developed in the 1980s to replace thermocouple and thermistor based temperature measurement systems. DTS technology is based on Optical Time-Domain Reflectometry (OTDR) and utilizes techniques originally derived from telecommunications cable testing. Today DTS provides a cost-effective way of obtaining hundreds, or even thousands, of highly accurate, high-resolution temperature measurements, DTS systems today find widespread acceptance in industries such as oil and gas, electrical power, and process control.
The underlying principle involved in DTS-based measurements is the detection of spontaneous Raman back-scattering. A DTS system launches a primary laser pulse that gives rise to two back-scattered spectral components. A Stokes component that has a lower frequency and higher wavelength content than the launched laser pulse, and an Anti-Stokes component that has a higher frequency and lower wavelength than the launched laser pulse. The Anti-Stokes signal is usually an order of magnitude weaker than the Stokes signal (at room temperature) and it is temperature sensitive, whereas the Stokes signal is almost entirely temperature independent. Thus, the ratio of these two signals can be used to determine the temperature of the optical fiber at a particular point. The time of flight between the launch of the primary laser pulse and the detection of the back-scattered signal may be used to calculate the special location of the scattering event within the fiber.
One problem involved in the operation of DTS systems is proper calibration. DTS technology derives temperature information from two back-scattered signals that are in different wavelength bands. The shorter wavelength signal is the Raman anti-Stokes signal, the longer one is usually the Raman Stokes signal. After the light from the primary source at λ1 is launched in a temperature sensing fiber, the scattered power arising from different locations within the optical fiber contained in the Stokes (λ1Stokes) and anti-Stokes (λAnti-Stokes) bands travel back to the launch end and gets detected by single or multiple detectors. As the Stokes and anti-Stokes signals travel, they suffer different attenuation profiles αStokes and αAnti-Stokes respectively, due to the difference in the wavelength band for these two signals. For proper temperature measurement a correction needs to be made so that the two signals exhibit the same attenuation.
One approach that has been used is to assume that the attenuation profile is exponentially decaying as a function of distance. This creates an exponential function with an exponent called the Differential Attenuation Factor (DAF) that is multiplied by the Stokes signal to adjust the attenuation profile to that of the anti-Stokes signal. The ratio of the resulting two signals is then used to derive temperature. The DAF is the difference in attenuation (αAS-αS) between two different wavelengths.
The assumption of a smooth exponential decay however is not always a reality. A number of factors can cause the actual attenuation to deviate from the exponential form. Localized mechanical stress or strain, fiber crimping, chemical attack (eg. hydrogen ingression) all can induce abnormalities, and some of these can change with time. It has been recognized in the industry that some form of continuous calibration is needed to reduce all of these irregularities.
U.S. Pat. No. 7,126,680 B2, Yamate et al. proposed using two additional light sources—one in the Stokes band of the primary source and the other in the anti-Stokes band of the primary source—to generate Rayleigh OTDR signals and time-correct the attenuation profile of the back-scattered signals. Therefore, Yamate et al. effectively propose removing the attenuation component from the back-scattered availability of desired light sources or the issue of cost have been obstacles to a practical implementation.
Some single source methods have been proposed in the past using Rayleigh and anti-Stokes bands (Farries—UK patent GB2183821—1987). One of the current inventors proposed a dual source approach in U.S. application Ser. No. 11/685,637. Each of these schemes have fairly slow response and are not fully automatic.
Double ended configurations (both ends of sensing fiber connected to DTS unit to cancel out common attenuations) have been used. These may double the length of sensing fiber and the sensing time, require an extra monitoring channel, and are not universally applicable in applications where space is limited.